The present invention is directed towards a capacitive sensor for sensing the amount of material in a container.
To date, a variety of sensors have been proposed for detecting the amount of toner in the toner cartridges of reprographic devices, such as printers and copiers. One type of such a sensor is a xe2x80x9ccapacitive sensor.xe2x80x9d A capacitive sensor (1) treats the toner material as part of a xe2x80x9ctoner-capacitor,xe2x80x9d and (2) detects changes in the toner level by sensing changes in the toner capacitance.
In such sensors, a toner-capacitor is typically formed by placing two conductive electrodes near the toner material. The two electrodes act as the two plates of the toner-capacitor. The toner material serves as one portion of the dielectric material of this capacitor while air serves as another portion of this dielectric material. Hence, the capacitance of the toner-capacitor depends on the toner level, and this capacitance decreases as the toner level decreases and is replaced with air.
FIG. 1 presents a prior art capacitive sensor 100 that treats the toner material as part of a toner-capacitor. The capacitive sensor 100 is similar to a sensor disclosed in U.S. Pat. No. 4,133,453. This sensor generates an oscillating signal, which has a frequency that varies with the capacitance of the toner-capacitor. This sensor uses the frequency of the generated oscillating signal to gauge the toner capacitance and thereby the toner level.
As shown in FIG. 1, the capacitive sensor 100 includes (1) a constant voltage source 125, (2) an oscillator 130, (3) a frequency-to-voltage converter 135, and (4) a comparator 140. The oscillator 130 includes a toner capacitor that connects to the voltage source 125. The oscillator generates an oscillating signal, whose frequency is dependent on the toner capacitance.
The frequency-to-voltage converter 135 receives the oscillating output signal, and generates an output voltage from the frequency of this signal. The 140 compares this output voltage with a reference voltage VREF. Based on this comparison, the sensor 100 determines whether the capacitance of the toner-capacitor has decreased below a referenced level. Such a decrease would indicate that the toner level has decreased below a threshold level.
FIG. 2 presents a more detailed view of the oscillator 130. As shown in this figure, the oscillator 130 includes (1) a toner-capacitor 105, (2) resistors 230 and 235, and (3) a 555-timer 200. The toner-capacitor 105 is formed by placing two electrodes 110 and 115 in a toner container. This capacitor""s second electrode 10 connects to ground, while its first electrode 115 connects to the constant voltage source 125 through resistors R1 (230) and RT (235).
The toner-capacitor""s first electrode 115 also connects to an input 245 of the 555-timer 200 to provide an input voltage. This timer includes a lower comparator 205, an upper comparator 210, a flip-flop 215, a discharge transistor 220, and an inverting output driver 225. The lower comparator 205 compares the input voltage (from the first electrode 115) with ⅓VCC, while the upper comparator 210 compares the input voltage with ⅔Vcc.
When the input voltage reaches ⅔VCC, the upper comparator sets the flip-flop 215 to output a high value. In turn, this high value (1) turns on the discharge transistor 220, and (2) causes the inverting output driver 225 to output a low value. When the input voltage reaches ⅓VCC, the lower comparator resets the flip-flop 215 to output a low value. This low value (1) cuts off the discharge transistor 220, and (2) causes the inverting output driver 225 to output a high value.
The operation of the oscillator 130 is as follows. Initially, the discharge transistor 220 of the timer 200 is off. This allows the first electrode 115 of toner-capacitor 105 to charge towards VCC through resistors 230 and 235. When the voltage on the first electrode 115 reaches ⅔VCC, the upper comparator 210 sets the flip-flop 215 to output a high value. This high value turns on the discharge transistor 220 and causes the inverting output driver 225 to output a low value. The discharge transistor 220, in turn, discharges the toner-capacitor 105 until the voltage on the first electrode 115 reaches ⅓VCC. At this time, the lower comparator 205 resets the flip-flop 215 to output a low value. This low value turns off the discharge transistor 220 and causes the inverting output driver 225 to output a high value. This oscillating process continues indefinitely, and results in an oscillating signal at the oscillator output 240.
The frequency of the oscillating output signal depends on how quickly the toner-capacitor charges to ⅔VCC and discharges to ⅓Vcc. Equation (1) below represents the frequency of the oscillating signal when the resistance, R1, of resistor 230 is much smaller than the resistance, RT, of resistor 235 (e.g., resistance R1 is 4.7 kxcexa9 while resistance RT is 47 kxcexa9).
xe2x80x83f0=0.722/(RT*CT).xe2x80x83xe2x80x83(1)
The frequency of the oscillating signal typically needs to be less than 20 kHz, because higher frequencies radiate more easily to the outside of the printer. This upper frequency constraint limits the amount of capacitance that sensor 100 can measure. When resistor 235 is 47 kxcexa9, a toner capacitance of 750 pF causes the output of oscillator 130 to have a frequency of 20.48 kHz. Hence, the toner capacitance cannot be much smaller than 750 pF, because otherwise the oscillating frequency would greatly exceed 20 kHz. Also, resistor 235 cannot be made arbitrarily large to reduce the oscillating frequency, because that would upset the bias currents at the inputs of the 555-timer 200.
To ensure that the toner capacitance stays larger than 750 pF, large-area electrodes or multiple electrodes in parallel pairs are used to form the toner capacitor 105. In addition, the toner-capacitor""s electrodes have to be placed within the container that stores the toner material, since the capacitance of a capacitor decreases as the distance between the capacitor""s electrodes increases. When the toner-capacitor""s electrodes are outside the toner container, the toner capacitance typically is less than 750 pF, and often falls within the sub-pico Farad range as the toner level decreases.
Consequently, the prior art sensor 100 cannot be used when its toner-capacitor electrodes are placed outside of the toner container, because cannot detect small toner capacitances while maintaining proper operational parameters. However, it is often desirable to place the electrodes outside the toner container of the toner cartridge, because placing electrodes inside cassettes adds cost to the consumable element rather than the more expensive printer engine.
Another disadvantage of the sensor 100 is that the toner-capacitor""s first electrode 115 is driven by the voltage source 125 through a high-impedance path (i.e., through high-impedance resistor 235). This high-impedance path makes the input voltage to the timer 200 susceptible to shunting capacitances. Thus, the wires and electrical elements connecting the toner-capacitor 105 and the voltage source 125 must be far enough away from neighboring conductive objects to avoid shunting capacitances and thereby allow accurate and repeatable measurements. This further reduces the possible positions of the toner-capacitor electrodes relative to other conducting parts of the printer and the toner cassette.
FIG. 3 presents another prior art sensor, which is similar to the sensors disclosed in U.S. Pat. Nos. 5,465,619 and 5,987,269. Sensor 300 uses the toner-capacitor to generate a signal, and then analyzes the amplitude of this signal to derive the toner capacitance CT and thereby the toner level. As shown in FIG. 3, sensor 300 includes (1) a toner-capacitor 305, (2) a power source 325, (3) an amplifier 330, (4) a rectifier 335, and (5) a comparator 340.
The toner-capacitor is formed by placing two electrodes 310 and 315 outside the toner container 320. The A.C. power source 325 drives the first electrode 310 of the toner-capacitor. This driving induces current flow between the amplifier 330 and the second electrode 315 of the toner-capacitor 305. The magnitude of this induced current depends on the capacitance CT of the toner-capacitor, and thereby on the amount of toner in the container 320.
The amplifier 330 amplifies the induced current and generates an A.C. voltage signal. The amplifier also maintains the second electrode 315 at virtual ground. The rectifier 335 then,converts the amplified A.C. voltage signal into a D.C. voltage. The comparator 340 compares the generated D.C. voltage with a reference voltage VREF, and this comparison indicates whether the toner level is less than a predetermined amount.
The sensor 300 can measure sub-pico Farad capacitances without the high frequency limitations of sensor 100. Thus, it can be used to measure the capacitance that results from placing the capacitor electrodes outside of the toner container. However, for two reasons, the sensor 300 has poor response in electrically noisy environments.
First, the toner-capacitor is susceptible to shunting capacitances because its first electrode 310 is driven by the high-impedance output of the power supply. The high impedance path to the toner-capacitor causes the current, which is intended for the toner capacitor, to divert to neighboring conductive objects. Hence, sensor 300 needs the wires and electrical elements connecting the first electrode 310 and the power source 325 to be isolated from neighboring conductive objects, in order to avoid shunting capacitances and thereby allow accurate measurements.
Second, the second electrode 315 is susceptible to direct induced currents from nearby fluctuating voltage potentials. Such fluctuating potentials can be due to (1) moving, conducting objects near the second electrode, or (2) stationary nearby objects that have fluctuating potentials. Hence, the second electrode 315 and connecting wires have to be far enough away from fluctuating voltage potentials, in order to allow the sensor to take accurate measurements. Sensor 300 is thus not able to measure small changes in capacitance within these printers without costly low-pass and band-pass filters, and other stabilizing features.
The noise sensitivity of the toner-capacitor electrodes 310 and 315 limits their positions to electrically quiet locations inside the printer. For example, sensor 300 cannot be placed near a conducting or rotating stir rod inside a toner cassette. However, it is often desirable to position at least one electrode inside the toner cartridge near the moving parts of the cartridge, because such positions typically provide the most sensitive toner level measurements.
In view of the foregoing, one of ordinary skill will understand that there is still a need in the art for a capacitive sensor that measures sub-pico Farad capacitances. Ideally, this sensor should be insensitive to unwanted shunting capacitances and should be able to operate in electrically noisy environments. There is also a need for a capacitance sensing circuit that is less expensive and requires looser tolerances on part specifications. There is a further need for a toner sensor that does not increase the cost or complexity of the toner cartridges.
One embodiment of the invention is a capacitive sensor for sensing the amount of material in a container. This embodiment includes an oscillator that generates an oscillating signal. This oscillator includes an integrating amplifier that is formed by (1) an operational amplifier that has a low-impedance output and a virtually-grounded input, and (2) a capacitor that has a capacitance which depends on the amount of material in the container. The capacitor connects between the low impedance output and the virtually-grounded input of the operational amplifier. In some embodiments, the operational amplifier includes a first amplifier and a second amplifier that are connected in series, with an output of the second amplifier fed back through the capacitor to the input of the first amplifier.
Another embodiment of the invention is a capacitive toner-level sensor that senses the amount of toner material in a toner cartridge of a reprographic device. This embodiment also includes an oscillator that generates an oscillating signal. The oscillator includes a toner-capacitor, a capacitance multiplier, and a resistance multiplier. The toner-capacitor has a capacitance that depends on the toner level in the container. The toner-capacitor also has a first electrode and a second electrode. The capacitance multiplier couples to the second electrode in order to drive this electrode to a negative voltage, while the resistance multiplier couples to the first electrode in order to reduce the current flow to and from the first electrode.
In some embodiments of the invention, one of the electrodes of the toner capacitor is a conductive component part of a toner cartridge, while the other electrode is positioned outside of the toner cartridge. In some of these embodiments, the conductive component part is a developer roller.